Speech analyzing systems match a received speech signal to a stored database of speech patterns. One system, a speech recognizer, interprets the speech patterns, or sequences of speech patterns to produce text. Another system, a vocoder, is a speech analyzer and synthesizer which digitally encodes an audio signal for transmission. The audio signal received by either of these devices often includes environmental noise. The noise acts to mask the speech signal, and can degrade the quality of the output speech of a vocoder or decrease the probability of correct recognition by a speech recognizer. It would be desirable to filter out the environmental noise to improve the performance of a vocoder or speech recognizer.